Fatigue Crack Paths 2003

Both materials were annealed for 2 h at 700°C to relieve internal stresses and

quenched to mineral oil. Furthermore, one set of the samples machined from the tube

was subjected to a solution treatment for 2 h at 1000°C, followed by oil quench and

precipitation annealing for 160 min. at 600°C in order to to form a fine Cr-Fe-(C)

precipitate [13] in the alloy.

Fatigue Crack Growth Tests

Fatigue crack growth experiments were performed on single edge notched (SEN) and

compact tension (CT) specimens. All samples were 5 m mthick. The S E N specimens

from the extruded tube were 75 m m× 13 m min dimensions. The C T specimens

machined from the hot rolled plate were 33 m mwide and 32 m mhigh. In all specimens,

the fatigue crack propagated in perpendicular to the rolling or extrusion direction. The

specimens were loaded in tension at room temperature on a computer–controlled

servohydraulic loading machine I N O V AZ U Z50. The frequency of loading was 10 Hz,

the stress ratio parameter R was in the range from 0.05 to 0.1.

The side of specimens was metalographically polished and crack length was

measured during fatigue test by means of a low power travelling microscope and also

by the potential method at alternative current with the frequency of 4 Hz, using

T E C H L ASBRT-2Kdevice, controlled by Fatigue Crack Growth Monitor software.

Scanning electron microscopes JEOLJSM840A (tungsten gun) and L E OGemini 2

(field-emission gun) were used for fractographic analysis.

R E S U L T S

Fatigue Crack Growth

Results of fatigue tests were plotted as a dependence between measured crack length a,

and elapsed number of loading cycles N. Subsequently, the dependence crack length a

vs. fatigue crack growth rate v was determined using secant method. Simultaneously,

(a)

(b)

T U B E(SEN)

P L A T E(CT)

600°C/160 min 7 120

700°C/120min Sample12

Figure 1. Fatigue crack growth curves for both materials.